Fuel cell system and control method thereof

ABSTRACT

A fuel cell system includes a fuel cell, a first combustor, a second combustor, a first heating gas return channel, a second heating gas return channel and a gas supplier. The fuel cell includes a solid electrolyte cell with an anode and a cathode. The first combustor supplies a heating gas to the cathode. The second combustor supplies a heating gas to the anode. The first heating gas return channel is arranged to mix at least some exhaust gas discharged from the cathode with the heating gas from the first combustor. The second heating gas return channel is arranged to mix at least some exhaust gas discharged from the cathode with the heating gas from the second combustor. The gas supplier is connected to the first heating gas return channel for supplying the exhaust gas from the cathode to mix with the heating gas of the first combustor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/322,038 filed on Nov. 22, 2011. U.S. patent application Ser.No. 13/322,038 is a U.S. National stage of International Application No.PCT/IB2010/001628, filed Jul. 2, 2010, which claims priority to JapanesePatent Application No. 2009-177746, filed on Jul. 30, 2009. The entiredisclosures of both the Japanese Patent Application No. 2009-177746 andthe U.S. patent application Ser. No. 13/322,038 are hereby incorporatedherein by reference.

BACKGROUND

1. Field of the Invention

The present invention generally relates to a fuel cell system and acontrol method for raising the temperature of a fuel cell used in thefuel cell system.

2. Background Information

A fuel cell system is an electric power generation system in whichhydrogen (serving as fuel) and air (serving as oxidizer) are supplied toa fuel cell to allow electrochemical reaction to take place in the fuelcell to generate electric power. One example of this type of fuel cellsystem is disclosed in Japanese Laid-Open Patent Application No.2005-166439. The fuel cell system disclosed in Japanese Laid-Open PatentApplication No. 2005-166439 uses a solid electrolyte fuel cell in whichan anode is provided on one side of a solid electrolyte, while a cathodeis provided on the other side. Air is supplied as oxidizing gas to thecathode while fuel gas is supplied to the anode. Power is generated byreacting the fuel gas with the air. The fuel cell system is configuredhaving a startup combustor for reforming or partially combusting fuelgas introduced from the exterior during startup and supplying theresulting gas as a reducing gas to the anode. An exhaust gas combustoris provided for burning the anode off-gas discharged from the anodeside, while a heat exchanger is provided for heating air with the heatproduced from the exhaust gas combustor.

SUMMARY

It has been discovered that in the fuel cell system disclosed inJapanese Laid-Open Patent Application No. 2005-166439, the anode off-gasdischarged from the anode is burned and the fuel cell is heated by theair that has been increased in temperature by the heat of the gas, butthe heat of the exhaust gas discharged from the cathode has not beenutilized. Also a concern exists that carbon deposition may be caused inthe anode by supplying rich combustion gas having a comparatively lowtemperature to the anode. This configuration does not take into accountthis carbon deposition.

One object of the present disclosure is to provide a fuel cell systemand/or method whereby the heat from the exhaust gas that is dischargedfrom the cathode is effectively utilized to raise the temperature of thefuel cell while avoiding partial damage and other problems caused by thetemperature changes, and to prevent carbon deposition in the anode.

In view of the state of the known technology, one aspect of the presentdisclosure is to provide a fuel cell system that at basically comprisesa fuel cell, a first combustor, a second combustor, a first heating gasreturn channel, a second heating gas return channel and a gas supplier.The fuel cell includes a solid electrolyte cell with an anode and acathode. The fuel cell is configured to generate power by reacting ahydrogen-containing gas and an oxygen-containing gas. The firstcombustor is arranged to selectively supply a heating gas to the cathodeof the fuel cell. The second combustor is arranged to selectively supplya heating gas to the anode of the fuel cell. The first heating gasreturn channel is arranged to mix at least some exhaust gas dischargedfrom the cathode with the heating gas from the first combustor such thata mixed heating gas of the exhaust gas from the cathode and the heatinggas from the first combustor is supplied to the cathode. The secondheating gas return channel is arranged to mix at least some exhaust gasdischarged from the cathode with the heating gas from the secondcombustor such that a mixed heating gas of the exhaust gas from thecathode and the heating gas from the second combustor is supplied to theanode. The gas supplier is connected to the first heating gas returnchannel for supplying the exhaust gas from the cathode to mix with theheating gas of the first combustor.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure:

FIG. 1 is a schematic block diagram of a configuration of a fuel cellsystem according to a first embodiment;

FIG. 2 is a schematic block diagram of a controller of the fuel cellsystem according to the first embodiment;

FIG. 3 is a flowchart of a fuel cell temperature-raising method executedby the controller of the fuel cell used in the fuel cell systemaccording to the first embodiment;

FIG. 4 is a schematic block diagram of a configuration of a fuel cellsystem according to a second embodiment;

FIG. 5 is a schematic block diagram of a controller of the fuel cellsystem according to the second embodiment;

FIG. 6 is a flowchart of a fuel cell temperature-raising method executedby the controller of the fuel cell used in the fuel cell systemaccording to the second embodiment;

FIG. 7 is a schematic block diagram of a configuration of a fuel cellsystem according to a third embodiment;

FIG. 8 is a schematic block diagram of a controller of the fuel cellsystem according to the third embodiment;

FIG. 9 is a flowchart of a fuel cell temperature-raising method executedby the controller of the fuel cell used in the fuel cell systemaccording to the third embodiment;

FIG. 10 is a schematic block diagram of a configuration of a fuel cellsystem according to a fourth embodiment;

FIG. 11 is a schematic block diagram of a controller of the fuel cellsystem according to the fourth embodiment; and

FIG. 12 is a flowchart of a fuel cell temperature-raising methodexecuted by the controller of the fuel cell used in the fuel cell systemaccording to the fourth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Selected embodiments will now be explained with reference to thedrawings. It will be apparent to those skilled in the art from thisdisclosure that the following descriptions of the embodiments areprovided for illustration only and not for the purpose of limiting theinvention as defined by the appended claims and their equivalents.

Referring initially to FIG. 1, a schematic block diagram of a fuel cellsystem A1 is illustrated in accordance with a first embodiment. As seenin FIG. 1, the fuel cell system A1 includes, among other things, acontroller B, an air blower 1, a fuel pump 2 and a fuel cell 10. In theillustrated embodiment, the fuel cell 10 is a solid oxide fuel cell(SOFC) in which an oxygen ion conductor (oxide ion conductor) is used asan electrolyte 11. In the illustrated embodiment, the electrolyte 11 hasan anode 12 provided on one side of the electrolyte 11 and a cathode 13provided on the other side of the electrolyte 11. In the illustratedembodiment, the electrolyte 11 has a plurality of solid electrolytecells 14 with the anode 12 and the cathode 13 being located on oppositesides of the solid electrolyte cells 14. Specifically, in theillustrated embodiment, the solid electrolyte cells 14 are stacked toform a cell stack 15 with the anode 12 and the cathode 13 being locatedon opposite sides of the cell stack 15. For the sake of illustration,the cell stack 15 is depicted in a simplified manner in FIG. 1 byshowing only a single cell of the solid electrolyte cells 14. Atemperature sensor 16 is disposed in the cell stack 15 for acquiringtemperature data of the cell stack 15. The temperature data acquired bythe temperature sensor 16 is inputted to the controller B.

Generally speaking, power in the fuel cell 10 is generated by reactingthe fuel gas with the air. The solid electrolyte cells 14 is an electricpower generation system that generates electric power by separatelysupplying hydrogen-containing gas, serving as fuel, andoxygen-containing gas, serving as oxidizer to allow electrochemicalreaction to take place in the fuel cell. In particular, in theillustrated embodiment, the fuel cell system A1 can use, for example,“ethane, butane, natural gas, and other suitable gases” as the“hydrogen-containing gas” that is supplied as fuel to the anode 12. Itis preferable to use ethanol, butanol, or another alcohol. However, incases of the fuel cell system A1 being used in vehicles such asautomobiles or other mobile units, gasoline, diesel oil, light oil, oranother liquid fuel, can be particularly useful in such cases. However,the fuel is not limited to these examples. Also, in the illustratedembodiment, the fuel cell system A1 uses “air” as an example of the“oxygen-containing gas” that is supplied to the as oxidizing gas to thecathode 13.

As also seen in FIG. 1, the fuel cell system A1 includes, among otherthings, a first combustor 20, a reformer 30, a heat exchanger 40, a gassupplier 50 and a third combustor 70. The air blower 1 is configured andarranged to supply fresh oxygen-containing gas to the first combustor 20and the reformer 30. The fuel pump 2 is configured and arranged tosupply fuel to the first combustor 20. The rotational speeds of the airblower 1 and the fuel pump 2 are controlled by the controller B so as toincrease and decrease their rotational speeds as needed. The controllerB of the fuel cell system illustrated in FIG. 1 is schematicallyillustrated in FIG. 2. In the illustrated embodiment, as discussedbelow, exhaust gas discharged from the anode 12 of the fuel cell systemA1 is effectively utilized to raise the temperature the fuel cell 10while avoiding partial damage and other problems caused by temperaturechanges in the fuel cell 10, and preventing carbon deposition.

The first combustor 20 performs the function of producinghigh-temperature heating gas. The high-temperature heating gas isproduced by mixing and burning a fuel-air mixture. The air is suppliedto the first combustor 20 through a supply pipe 1 a that is fluidlyconnected between the intake side of the first combustor 20 and the airblower 1. The fuel is supplied to the first combustor 20 through asupply pipe 2 a that is fluidly connected between the intake side of thefirst combustor 20 and the fuel pump 2.

On the discharge side of the first combustor 20, a supply pipe 20 a isfluidly connected between the discharge side and the intake side of thecathode 13 of the fuel cell 10. The supply pipe 20 a is designed tosupply the heating gas produced by the first combustor 20 to the cathode13. On the discharge side of the cathode 13 is disposed a discharge pipe13 a for discharging the exhaust heating gas discharged from the cathode13 out of the fuel cell system A1. Spanning between the supply pipe 20 aand the discharge pipe 13 a is a return channel or pipe 17. In thisembodiment, the return pipe 17 constitutes a first exhaust heating gasreturn channel. The return pipe 17 is configured and arranged for mixingsome of the exhaust heating gas discharged from the cathode 13 withheating gas supplied from the first combustor 20 to the cathode 13.Specifically, the return starting end of the return pipe 17 isinterconnected with the discharge pipe 13 a, and the return terminal endis interconnected with the supply pipe 20 a. The gas supplier 50 isdisposed in the return channel 17. The gas supplier 50 performs thefunction of supplying exhaust heating gas flowing into the return pipe17 to the cathode 13. In the present embodiment, the gas supplier 50 isan air blower. Specifically, mixed heating gas is produced by mixing theexhaust heating gas discharged from the cathode 13 with the heating gassupplied from the first combustor 20, and the resulting mixed heatinggas is supplied to the cathode 13 by the return pipe 17 and the gassupplier 50. A temperature sensor 19 is disposed in the gas supplier 50for acquiring temperature data of the exhaust heating gas supplied fromthe gas supplier 50. The temperature data acquired by this temperaturesensor 19 is inputted to the controller B.

The reformer 30 is configured and arranged for reforming fuel gassupplied to the anode 12 of the fuel cell 10 in a normal operation modedescribed hereinafter. A supply pipe 2 b is fluidly connected betweenthe intake side of the reformer 30 and the outlet side of the fuel pump2 for supplying fuel to the reformer 30. A supply pipe 1 b is fluidlyconnected between the intake side of the reformer 30 and the air blower1 for supplying air to the reformer 30. A supply pipe 30 a is fluidlyconnected between the supply side of the reformer 30 and the intake sideof the anode 12 such that reformed fuel gas supplied from the reformer30 is supplied to the anode 12. The reformer 30 can be provided with atemperature sensor 30 b for acquiring temperature data of the reformer30 as needed and/or desired.

On the discharge side of the anode 12 is disposed a discharge pipe 12 afor supplying discharged exhaust fuel gas to the third combustor 70. Thethird combustor 70 performs the function of producing high-temperatureheating gas by mixing and burning an air-fuel mixture of fuel and eitherfresh air or exhaust heating gas discharged from the anode 12. Thedischarge pipe 12 a is fluidly connected between the intake side of thethird combustor 70 and the discharge side of the anode 12. Between thedischarge side of the third combustor 70 and the intake side of the heatexchanger 40 is disposed a supply pipe 12 b, which is a heating gassupply channel for supplying the heating gas produced by the thirdcombustor 70 to the heat exchanger 40.

The heat exchanger 40 is disposed adjacent to the reformer 30 so thatheat exchange occurs between them. The heat exchanger 40 is designed tobe supplied with some of the combustion gas resulting from the exhaustfuel gas supplied from the anode 12 through the supply pipe 12 b beingburned by the third combustor 70. On the discharge side of the heatexchanger 40 is disposed a discharge pipe 40 a for discharging exhaustfuel gas out of the system after the gas has been used in heat exchange.

In the present embodiment, during a temperature increase for raising thetemperature of the fuel cell 10 to an operable temperature (duringstartup or temperature-raising mode), the reformer 30, the heatexchanger 40, and the third combustor described above do not operate,and fuel gas is not supplied to the anode 12. Thus, thetemperature-raising mode is only performed up until the temperature ofthe fuel cell 10 reaches its prescribed operating temperature.

In the illustrated embodiment, the controller B includes a microcomputerwith a CPU (central processing unit), an interface circuit, storagedevices such as a ROM (Read Only Memory) device and a RAM (Random AccessMemory) device and other conventional components (not shown). Themicrocomputer of the controller B is programmed to control the othercomponents of the fuel cell system A1 as discussed below. The memorycircuit stores processing results and control programs that are run bythe processor circuit. The internal RAM of the controller B storesstatuses of operational flags and various control data. The internal ROMof the controller B stores various prescribed data for variousoperations.

The controller B includes one or more programs that are used inoperation of the fuel cell system A1. By executing these programs, thecontroller B performs the following functions: (1) measuring thetemperature of the exhaust heating gas flowing through the return pipe17; (2) measuring the flow rate of exhaust heating gas flowing throughthe return pipe 17; (3) setting the fuel and air flow rates of the fueland the air supplied to the first combustor 20 based on the flow rateand temperature of the exhaust heating gas flowing through the returnpipe 17, so that the heating gas supplied to the cathode 13 reaches apredetermined temperature; (4) supplying fuel and oxygen-containing gaswith a set flow rate to the first combustor 20; (5) measuring thetemperature of the fuel cell 10; (6) setting the temperature of theheating gas supplied to the cathode 13 of the fuel cell 10 on the basisof the temperatures of the exhaust heating gas flowing through the fuelcell 10 and the return pipe 17; (7) determining whether or not thetemperature of the fuel cell 10 has reached a predetermined value; and(8) switching from temperature-raising mode to normal operation modewhen temperature of the fuel cell 10 is determined to have reached thepredetermined value. It will be apparent to those skilled in the artfrom this disclosure that the precise structure and algorithms for thecontroller B can be any combination of hardware and/or software thatwill carry out the described functions.

The programming and/or hardware of the controller B used to perform thefunction of measuring the temperature of the exhaust heating gas flowingthrough the return pipe 17 is referred to as a “first exhaust heatinggas temperature measurement section B1.” In the present embodiment, thetemperature of the exhaust heating gas is measured based on thetemperature data acquired by the temperature sensor 19. The programmingand/or hardware of the controller B used to perform the function ofmeasuring the flow rate of exhaust heating gas flowing through thereturn pipe 17 is referred to as the “first exhaust heating gas flowrate measurement section B2.” The first exhaust heating gas flow ratemeasurement section B2 measures the gas flow rate of the exhaust heatinggas from the rotational speed of the blower and the quantity of gas thatcan be blown by one rotation of the blower, according to the design ofthe gas supplier 50. The programming and/or hardware of the controller Bused to perform the function of setting the flow rates of the fuel andthe air supplied to the first combustor 20 is referred to as the “flowrate setting section B3.” The term “predetermined temperature” refers toa temperature at which the fuel cell 10 will not be damaged by thermalshock, based on the current temperature of the fuel cell 10. Theprogramming and/or hardware of the controller B used to perform thefunction of supplying fuel and oxygen-containing gas with a set flowrate to the first combustor 20 is referred to as the “fuel gas supplysection B4.” In the present embodiment, supply is performed by rotatablyand drivably controlling the air blower 1 and the fuel pump 2. Theprogramming and/or hardware of the controller B used to perform thefunction of measuring the temperature of the fuel cell 10 is referred toas the “cell temperature measurement section B5.” In the presentembodiment, the temperature of the fuel cell 10 is measured based on thetemperature data acquired by the temperature sensor 16. The programmingand/or hardware of the controller B used to perform the function ofsetting the temperature of the heating gas supplied to the cathode 13 isreferred to as the “gas temperature setting section B6.” In the presentembodiment, the temperature of the heating gas is set to increase overtime to a target temperature. The programming and/or hardware of thecontroller B used to perform the function of determining whether or notthe temperature of the fuel cell 10 has reached a predetermined value isreferred to as the “cell temperature determination section B7.” Theprogramming and/or hardware of the controller B used to perform thefunction of switching from temperature-raising mode to normal operationmode when temperature of the fuel cell 10 is determined to have reachedthe predetermined value is referred to as the “mode switching sectionB8.”

As used herein, the term “temperature-raising mode” refers to the actionof raising the temperature of the fuel cell 10 to an operabletemperature as described above. As used herein, the term “normaloperation mode” refers to an operation state in the fuel cell 10 hasreached the operable temperature for inducing power generation in thefuel cell 10.

The temperature-raising method of the fuel cell used in the fuel cellsystem A1 having the configuration described above is described withreference to FIG. 3. FIG. 3 is a flowchart showing thetemperature-raising method of the fuel cell used in the fuel cell systemA1. The temperature-raising method used in the fuel cell system A1includes at least measuring the temperature of the exhaust heating gasflowing through the first exhaust heating gas return channel or pipe 17,setting the flow rates of the oxygen-containing gas and the fuel burnedby the first combustor 20 so that the new heating gas supplied from thefirst combustor 20 to the cathode 13 reaches a predeterminedtemperature, and supplying the fuel and oxygen-containing gas having setflow rates to the first combustor 20. The details thereof are asfollows.

Referring to the flow chart of FIG. 3, the process will now bediscussed. In step S1, the process of raising the temperature of thefuel cell 10 for a startup operation is started, and then the processadvances to step S2.

In step S2, the exhaust heating gas (exhaust lean combustion gas)discharged from the cathode 13 is passed through the return pipe 17,with the circulating return supply being performed at a constant flowrate.

Predetermined amounts of fuel and air are supplied to and burned in thefirst combustor 20 to produce new heating gas, which mixes with theexhaust heating gas supplied back through the return pipe 17 to producemixed heating gas of a predetermined temperature, which is supplied tothe cathode 13.

In step S3, the temperature of the exhaust heating gas circulatedthrough the return pipe 17 is measured and the temperature of the fuelcell 10 is measured.

In step S4, the temperature of the new heating gas supplied to the fuelcell 10 is set. At this time, the heating gas of a predeterminedtemperature is set to a temperature at which the fuel cell 10 describedabove is not damaged by thermal shock, based on the current temperatureof the fuel cell 10. This predetermined temperature is appropriately setin view of the heat capacity of the fuel cell 10 and the flow rate ofthe supplied heating gas, i.e., the heat capacity of the heating gas.

In step S5, the amount of heat needed in order to raise the temperatureof the fuel cell 10 to the predetermined temperature is calculated fromthe flow rate and specific heat of the exhaust heating gas circulatedback, and the flow rates of the fuel and air needed in the firstcombustor 20 in order to produce this amount of heat are determined.Since the heating gas is supplied to the cathode 13, the heating gas ispreferably a lean combustion gas having oxidative properties.Specifically, the gas undergoes lean combustion at an air-fuel ratio of1 to 1.2.

In step S6, the amounts of fuel and air calculated above are supplied tothe first combustor 20.

In step S7, the heating gas produced by the first combustor 20 is mixedwith the exhaust heating gas, and heating gas (mixed heating gas) of apredetermined temperature is supplied to the fuel cell 1. This mixedheating gas thereby raises the temperature of the fuel cell 10.

As previously described, since the temperature of the heating gassupplied to the fuel cell 10 is set according to the temperature of thefuel cell 10, the temperature of the fuel cell 10 is raised and thetemperature of the supplied heating gas is also gradually set to aprogressively higher temperature. The temperature of the mixed heatinggas supplied to the fuel cell 10 is assigned an upper limit temperaturein consideration of the heat resistance of the structural members. Forexample, in the present embodiment, the upper limit temperature is 800°C. Specifically, the set temperature of the heating gas graduallyincreases up to 800° C., after which the temperature of the heating gassupplied to the fuel cell 10 will be continued to be maintained at 800°C.

The exhaust heating gas discharged from the fuel cell 10 provides heatto the fuel cell 10, and the gas is also discharged at approximately thesame temperature as the fuel cell 10. Therefore, since the temperatureof the circulating exhaust heating gas rises along with the temperatureincrease in the fuel cell 10, the amount of combustion in the firstcombustor 20 is regulated according to the difference between the settemperature of the heating gas supplied to the fuel cell 10 and thetemperature of the circulating exhaust heating gas, the amount of theheating gas mixing with the circulating exhaust heating gas, and theamount of the mixed gas being supplied to the fuel cell 10. In this way,the heating gas is supplied to raise the temperature of the fuel cell 10until the fuel cell 10 reaches an operable temperature.

In step S8, a decision is made as to whether or not the fuel cell 10 hasreached the operable temperature. Once it has been determined that theoperable temperature of the fuel cell 10 has been reached, the processadvances to step S9. Otherwise, the process returns to step S2 until theoperable temperature of the fuel cell 10 has been reached.

In step S9, the heating temperature-raising operation is ended, and thenormal operation mode is reinstated. According to the configurationdescribed above, since circulated fuel gas (exhaust heating gas) isdischarged from the cathode at room temperature or higher, a lesseramount of heat, i.e., a lesser amount of burned fuel is needed in orderto produce the fuel gas at the same flow rate in comparison with usingnew or fresh air as the normal secondary air. Thus, fuel consumptionduring temperature elevation can be greatly reduced.

To consider utilizing waste heat, it would be possible to recover onlywaste heat with the heat exchanger without circulating exhaust fuel gas.However, since the heat exchanger itself has a low temperature during atemperature elevation process, first a certain amount of heat will beused to heat up the heat exchanger. If a startup operation is presumedto occur suddenly (i.e., the temperature rises suddenly), a large amountof combustion gas will be supplied to the fuel cell, and an extremelylarge heat exchanger will be needed in order to recover waste heat fromthe large amount of combustion gas. Therefore, the heat capacity of theheat exchanger increases, and even if the heat exchanger is utilized forthe purpose of recovering waste heat during temperature elevation, therecovery rate of waste heat will not increase because of the amount ofheat required to preheat the heat exchanger.

Referring now to FIGS. 4 to 6, a fuel cell system A2 in accordance witha second embodiment will now be explained. In view of the similaritybetween the first and second embodiments, the parts of the fuel cellsystem A2 of the second embodiment that are identical to the parts ofthe first embodiment will be given the same reference symbols as theparts of the first embodiment. Moreover, the descriptions of the partsof the second embodiment that are identical to the parts of the firstembodiment have been omitted for the sake of brevity. FIG. 4 is aschematic block diagram showing the configuration of the fuel cellsystem A2 according to the second embodiment. FIG. 5 is a schematicblock diagram showing the functions of the controller B constitutingpart of the fuel cell system A2 according to the second embodiment. FIG.6 is a flowchart showing the method of raising the temperature of thefuel cell 10 that is used in the fuel cell system A2. In addition to theconfiguration shown in the fuel cell system A1 according to the firstembodiment described above, the fuel cell system A2 according to thesecond embodiment also includes a second combustor 60, a flow rateregulation valve 61 and a temperature sensor 62. Also the reformer 30,the heat exchanger 40, and the third combustor 70 are not used in thisembodiment.

The second combustor 60 performs the function of producinghigh-temperature heating gas. The second combustor 60 mixes and burningthe air supplied through the supply pipe 1 b from the air blower 1 withthe fuel supplied through the supply pipe 2 b from the fuel pump 2 toproduce the high-temperature heating gas. On the discharge side of thesecond combustor 60 is fluidly connected to a supply pipe 60 a forsupplying the produced heating gas to the anode 12 of the fuel cell 10.The flow rate regulation valve 61 is disposed in the supply pipe 13 a. Areturn channel or pipe 61 a is fluidly connected between the flow rateregulation valve 61 and the supply pipe 60 a. In this embodiment, thereturn pipe 61 a constitutes a second exhaust heating gas returnchannel.

The flow rate regulation valve 61 is operatively connected to the outputside of the controller B so that the controller B selectively opens andcloses the flow rate regulation valve 61. Specifically, according to anopen/close drive signal outputted from the controller B, the flow rateregulation valve 61 directs an appropriate amount of the exhaust heatinggas to flow through the return pipe 61 a. More specifically, the returnpipe 61 a is formed in order to circulate exhaust heating gas dischargedfrom the cathode 13 to the anode 12. In particular, at least some of theexcess exhaust heating gas is not circulated back to the cathode 13 andredirected to mix with the heating gas produced by the second combustor60. In this way, a mixture of the exhaust heating gas discharged fromthe cathode 13 and the heating gas produced by the second combustor 60are introduced into the anode 12.

The temperature sensor 62 is used to acquire temperature data of exhaustheating gas flowing through the return pipe 61 a. The temperature sensor62 is connected to the input side of the controller B. In other words,acquired temperature data of exhaust heating gas flowing through thereturn pipe 61 a is inputted into the controller B.

In this embodiment, the controller B includes one or more programs thatare used in operation of the fuel cell system A2. Similar to the firstembodiment as discussed above, by executing these programs, thecontroller B performs the functions of the first exhaust heating gastemperature measurement section B1, the first exhaust heating gas flowrate measurement section B2, the flow rate setting section B3, the fuelgas supply section B4, the cell temperature measurement section B5, thegas temperature setting section B6, the cell temperature determinationsection B7 and the mode switching section B8. However, in thisembodiment, in addition to theses functions, the controller B alsoperforms the following functions: (1) measuring the flow rate of theexhaust heating gas supplied to the anode 12; (2) measuring thetemperature of the exhaust heating gas; and (3) setting the flow ratesof the fuel and air burned in the second combustor 60 so that a steam(e.g., water vapor) to carbon ratio (S/C ratio) and the temperature ofthe fuel gas supplied to the anode 12 reach predetermined values basedon the flow rate and temperature of the exhaust heating gas dischargedfrom the cathode 13 and supplied to the anode 12.

The programming and/or hardware of the controller B used to perform thefunction of measuring the flow rate of the exhaust heating gas suppliedto the anode 12 through the return pipe 61 a is referred to as the“second exhaust heating gas flow rate measurement section B9.” Theprogramming and/or hardware of the controller B used to perform thefunction of measuring the temperature of the exhaust heating gas flowingthrough the return pipe 61 a is referred to as the “second exhaustheating gas temperature measurement section B10.” The programming and/orhardware of the controller B used to perform the function of setting theflow rates of the fuel and air burned in the second combustor 60 so thatthe S/C ratio and the temperature of the fuel gas supplied to the anode12 reach predetermined values is referred to as the “second flow ratesetting section B11.”

The method of raising the temperature of a fuel cell using the fuel cellsystem A2 having the configuration described above is described withreference to FIG. 6. FIG. 6 is a flowchart showing the method of raisingthe temperature of the fuel cell used in the fuel cell system A2.

In the present embodiment, the flow rates of the fuel and the airsupplied to the first combustor 20 are regulated so that the mixedheating gas supplied to the cathode 13 reach a predetermined temperaturebased on the flow rate and temperature of the exhaust heating gasflowing through the return pipe 17, which is similar to the fuel cellsystem A1 described above.

In step Sa1, the process of raising the temperature of the fuel cell 10for a startup operation is started, and the process advances to stepSa2.

In step Sa2, the flow rate is set for the delivery of exhaust heatinggas (exhaust lean combustion gas) discharged from the cathode 13 to thereturn pipe 61 a. Specifically, the exhaust heating gas supplied to theanode 12 has oxidation reducing properties in order to prevent oxidationof the anode 12. The reducing exhaust heating gas is provided with acertain amount of water vapor so as not to cause carbon deposition onthe anode 12. The reducing exhaust heating gas is also supplied whilebeing adjusted to a predetermined temperature so as not to cause thermalshock in the fuel cell 10. To accomplish these results, exhaust heatinggas discharged from the cathode 13 without being recirculated throughthe cathode 13 is used.

As described above, the exhaust heating gas flowing in the return pipe61 a has a low oxygen concentration. By mixing the exhaust heating gasof the return pipe 61 a with new heating gas produced from richcombustion in the second combustor 60 prior being introduced into theanode 12, the resulting mixed heating gas will have oxidation reducingproperties. Since the exhaust heating gas discharged from the cathode 13also contains a high concentration of water vapor, it is possible toprovide a water vapor concentration sufficient to prevent carbondeposition caused by the mixed heating gas in the anode 12. By using theexhaust heating gas supplied from the cathode 13 as thetemperature-regulating gas of the new heating gas produced by the secondcombustor 60, it is possible to supply the anode 12 with exhaust heatinggas that has reducing properties, no risk of carbon deposition, and alsothe desired temperature. It is also possible to appropriately set thedelivered amount of excess exhaust heating gas discharged without beingcirculated through the return pipe 17. To prevent oxidation of the anode12, fuel gas having the minimum required reducing properties ispreferably supplied. Thus, the flow rate of exhaust heating gas suppliedfrom the cathode 13 is preferably set to a small amount. Furthermore, incases in which the temperature of the fuel cell 10 is raised suddenly,the supply of a large amount of heating gas is efficient to the anode 12as well, and the flow rate of exhaust heating gas supplied from thecathode 13, is therefore set to a high rate.

As with cases with the cathode 13, the amount of combustion in thesecond combustor 60 needed in order to raise the temperature of theexhaust heating gas supplied from the cathode 13 to the predeterminedtemperature is set based on the predetermined temperature of the heatinggas supplied to the fuel cell 10. The predetermined temperature of theheating gas supplied to the anode 12 can be set independent of thecathode 13, but is preferably set to approximately the same settemperature as the cathode 13 in order to avoid thermal shock to thefuel cell 10. The amount of combustion in the second combustor 60 is setaccording to the amount of heat needed to raise the temperature of theexhaust heating gas supplied from the cathode 13. The amount ofcombustion in the second combustor 60 is also set in view of thecomposition of the mixed heating gas. Specifically, in order for themixed heating gas to have reducing properties, consideration is given tohow much unburned fuel is to be included, and also to how much watervapor is needed in the unburned fuel in order to prevent carbondeposition. Therefore, rich combustion is performed in the secondcombustor 60, but combustion is performed with the air-to-fuel ratiokept between less than 1 and the combustion limit (about 0.2 in the caseof gasoline).

As described above, the exhaust heating gas that does not circulatethrough the return pipe 17 is mixed with the new heating gas produced bythe second combustor 60 disposed on the upstream side of the anode 12until the fuel cell 10 reached the predetermined temperature. In thisway, heating gas having a temperature that does not cause thermal shockto the fuel cell 10 and which has reducing properties that eliminate therisk of carbon deposition in the cathode 13 is supplied to the fuel cell10 to raise the temperature.

In step Sa3, the temperature and composition of the exhaust heating gasdischarged from the cathode 13 and directed toward the anode 12 aredetected, measured and stored. In the present embodiment, thetemperature of exhaust heating gas directed toward the anode 12 isdetected by the temperature sensor 62. The temperature sensor 62 isdisposed in the flow rate regulation valve 61, but the temperaturedetected by the temperature sensor 19 disposed on the discharge side ofthe gas supplier 50 can be used as a substitute for the temperaturesensor 62.

The flow rate regulation valve 61 includes a measuring device formeasuring the composition of the exhaust heating gas. The composition ofthe exhaust heating gas is measured by this measuring device. However,the composition can also be estimated from the combustion conditions(air-to-fuel ratio) in the first combustor 20 because the compositiongradually approaches the heating gas composition produced in the firstcombustor 20 as described above. In other words, a configuration can beused in which a gas composition estimation section is provided forestimating the composition of the exhaust heating gas on the basis ofthe combustion conditions (air-to-fuel ratio) in the first combustor 20.

In step Sa4, the temperature of the heating gas supplied to the anode 12is set.

In step Sa5, the amount of combustion in the second combustor 60 is setbased on the flow rate and temperature of the exhaust heating gas.

In step Sa6, fuel and air are supplied to the second combustor 60.

In step Sa7, new heating gas supplied from the second combustor 60 andexhaust heating gas discharged from the cathode 13 are mixed andsupplied to the anode 12.

In step Sa8, a decision is made as to whether or not the fuel cell 10has reached the operable temperature. Once the operable temperature isdetermined to have been reached, the process advances to step Sa9.Otherwise, if the operable temperature has not been reached, then theprocess returns to step Sa2.

In step Sa9, the heating and temperature-raising operation is ended, andthe system transitions to the normal operation mode.

Referring now to FIGS. 7 to 9, a fuel cell system A3 in accordance witha third embodiment will now be explained. In view of the similaritybetween this third embodiment and the prior embodiments, the parts ofthe fuel cell system A3 of the third embodiment that are identical tothe parts of the prior embodiments will be given the same referencesymbols as the parts of the prior embodiments. Moreover, thedescriptions of the parts of the third embodiment that are identical tothe parts of the prior embodiments have been omitted for the sake ofbrevity. FIG. 7 is a schematic block diagram showing a configuration ofthe fuel cell system A3 according to the third embodiment. FIG. 8 is aschematic block diagram of the controller B of the fuel cell system A3according to the third embodiment. FIG. 9 is a flowchart showing a fuelcell temperature-raising method executed by the controller B of the fuelcell 10 that is used in the fuel cell system A3.

In addition to the configuration shown in the fuel cell system A1according to the first embodiment described above, the fuel cell systemA3 according to the third embodiment also has a configuration providedwith the flow rate regulation valves 61 of the second embodiment, thetemperature sensor 62 of the second embodiment and a flow rateregulation valve 71.

A supply pipe 2 b is fluidly connected between the intake side of thereformer 30 and the fuel pump 2. Also a supply pipe 30 a is fluidlyconnected between the discharge side of the reformer 30 and the anode12. The flow rate regulation valve 61 is disposed between the supplypipe 13 a, and the return pipe 61 a. The return pipe 61 a is fluidlyconnected between the flow rate regulation valve 61 and the supply pipe30 a. In other words, the return pipe 61 a is formed for supplying tothe anode 12 at least some of the excess exhaust heating gas dischargedfrom the cathode 13 that is not otherwise circulated back to the cathode13.

The flow rate regulation valve 71 is provided in the return pipe 61 a.The flow rate regulation valve 71 is designed so that a supply pipe 71 ais fluidly connected between the valve and the intake side of thereformer 30 and exhaust heating gas can be delivered to the anode 12 andthe reformer 30. The supply pipe 71 a constitutes a third exhaustheating gas return channel for supplying back to the reformer 30 atleast some of the exhaust heating gas discharged from the cathode 13.The flow rate regulation valve 71 is connected to the output side of thecontroller B so as to be selectively opened and closed by opening andclosing drive signals outputted from the controller B.

In this embodiment, the controller B includes one or more programs thatare used in operation of the fuel cell system A3. Similar to the firstembodiment as discussed above, by executing these programs, thecontroller B performs the functions of the first exhaust heating gastemperature measurement section B1, the first exhaust heating gas flowrate measurement section B2, the flow rate setting section B3, the fuelgas supply section B4, the cell temperature measurement section B5, thegas temperature setting section B6, the cell temperature determinationsection B7 and the mode switching section B8. However, in thisembodiment, in addition to theses functions, the controller B alsoperforms the following functions: (1) determining whether or not thetemperature of the reformer 30 has reached an operating temperature; (2)setting the flow rates of fuel and air to the reformer 30 on the basisof the temperature and the delivery amount of the exhaust heating gasdischarged from the cathode 13 that is supplied back to the reformer 30via the supply pipe 71 a when it has been determined that thetemperature of the reformer 30 has reached the operating temperature;and (3) supplying exhaust heating gas having this set flow rate to thereformer 30.

The programming and/or hardware of the controller B used to perform thefunction of determining whether or not the temperature of the reformer30 has reached an operating temperature is referred to as the “operationtemperature determination section B12.” The reformer 30 is provided withthe temperature sensor 30 b for acquiring temperature data of thereformer 30. The programming and/or hardware of the controller B used toperform the function of setting the flow rates of fuel and air to thereformer 30 is referred to as the “reformer flow rate setting sectionB13.” The programming and/or hardware of the controller B used toperform the function of supplying exhaust heating gas with this set flowrate to the reformer 30 is referred to as the “reformer gas supplysection B14.”

The method of raising the temperature of the fuel cell using the fuelcell system A3 having the above-described configuration is describedwith reference to FIG. 9. FIG. 9 is a flowchart showing the method ofraising the temperature of the fuel cell used in the fuel cell systemA3.

In the present embodiment, the increase and decrease of the flow ratesof hydrogen-containing gas and air supplied to the first combustor 20are regulated based on the flow rate and temperature of the exhaustheating gas flowing through the return pipe 17 so that the mixed heatinggas supplied to the cathode 13 reaches a predetermined temperature, aswith the fuel cell system A1 described above.

In step Sc1, the process of raising the temperature of the fuel cell 10for a startup operation is started, and the process advances to stepSc2.

In step Sc2, the reformer 30 is pre-heated by the exhaust heating gasproduced by the third combustor 70. The exhaust heating gas suppliedfrom the cathode 13 is applied to the reforming reaction in the reformer30, and carbon deposition in the cathode 13 is prevented by the reformedexhaust heating gas. Temperature-adjusting gas of the reformed heatinggas is also delivered upstream of the reformer 30 as well as being usedin order to mix with the reformed heating gas downstream of the reformer30.

In step Sc3, a decision is made as to whether or not the reformer 30 hasreached the operating temperature. Once the operable temperature isdetermined to have been reached, the process advances to step Sc4 if thereformer 30 is determined to have reached the operating temperature.Otherwise, if the operable temperature has not been reached, then theprocess returns to step Sc2.

In step Sc4, the flow rates of fuel and air to the reformer 30 are setfrom the delivery amount and the temperature of the reformer 30.

In step Sc5, fuel, air, and exhaust heating gas is supplied to thereformer 30.

First, to bring the reformer 30 to an operable temperature (theoperating temperature), fuel, air, and exhaust heating gas are suppliedto and mixed in the third combustor 70 to produce heating gas. Thisheating gas is supplied to a heat exchanger 40 provided in order topre-heat the reformer 30. Thus, the temperature of the reformer 30 israised. After the reformer 30 has reached the operating temperature,exhaust heating gas supplied from the cathode 13 and fuel are suppliedto the reformer 30. In this way, reformed gas is produced.

Since a minuscule amount of oxygen and a large amount of water vapor areincluded in the exhaust heating gas supplied from the cathode 13,reformed gas is produced in the reformer 30 by a partial oxidizingreaction and a water vapor reforming reaction. Since the partialoxidizing reaction is exothermic and the water vapor reforming reactionis endothermic, a balance between the rates of the two reactions is keptin order to stably operate the reformer 30, i.e., in order to keep thereformer 30 in a predetermined temperature range. Therefore, air issupplied as necessary to the reformer 30 in order to increase the rateof the partial oxidizing reaction.

After the reformed exhaust heating gas has been supplied to the anode12, the unburned fuel component included in the discharged reformed gasis burned in the third combustor 70, whereby high-temperature fuel gascan be produced and supplied as temperature-regulating gas of thereformer 30 to the heat exchanger 40.

The reformer 30 can be operated in a stable manner within thepredetermined temperature range by achieving a balance between thereaction rate in the reformer 30 and the heat from the exhaust heatinggas. The exhaust heating gas supplied from the cathode 13 is divided bythe flow rate adjustment valve 71 provided upstream of the reformer 30into a flow rate supplied to the reformer 30 for the reforming reactionand a flow rate supplied downstream of the reformer 30 in order toregulate the temperature of the reformed gas.

Similar to the fuel cell system A2 described above, the exhaust heatinggas after mixing is reducing reformed gas containing water vapor andhaving no risk of carbon deposition, and delivered amount and the amountof reformed gas produced in the reformer 30, i.e., the amount of fuelsupplied to the reformer 30 is regulated so that the predeterminedtemperature is reached.

In step Sc6, the reformed mixed heating gas is supplied to the anode 12.

In step Sc7, a decision is made as to whether or not the fuel cell 10has reached the operable temperature. Once the operable temperature isdetermined to have been reached, the process advances to step Sc8.Otherwise, if the operable temperature has not been reached, then theprocess returns to step Sc4.

In step Sc8, the heating and temperature-raising operation is ended, andthe system transitions to the normal operation mode.

Referring now to FIGS. 10 to 12, a fuel cell system A4 in accordancewith a fourth embodiment will now be explained. In view of thesimilarity between this fourth embodiment and the prior embodiments, theparts of the fuel cell system A4 of the fourth embodiment that areidentical to the parts of the prior embodiments will be given the samereference symbols as the parts of the prior embodiments. Moreover, thedescriptions of the parts of the fourth embodiment that are identical tothe parts of the prior embodiments have been omitted for the sake ofbrevity. FIG. 10 is a schematic block diagram showing a configuration ofthe fuel cell system A4 according to the fourth embodiment. FIG. 11 is aschematic block diagram of the controller B of the fuel cell system A4according to the fourth embodiment. FIG. 12 is a flowchart showing afuel cell temperature-raising method executed by the controller B of thefuel cell 10 that is used in the fuel cell system A4.

The fuel cell system A4 according to the fourth embodiment has theconfiguration shown in the fuel cell system A1 according to the previousfirst embodiment, to which a temperature sensor 63 is provided. Thetemperature sensor 63 is arranged for measuring the temperature of themixed heating gas supplied to the cathode 13 of the fuel cell 10. Thetemperature sensor 63 is connected to the input side of the controllerB. In other words, acquired temperature data of exhaust heating gas isinputted into the controller B.

In this embodiment, the controller B includes one or more programs thatare used in operation of the fuel cell system A4. Similar to the firstembodiment as discussed above, by executing these programs, thecontroller B performs the functions of the first exhaust heating gastemperature measurement section B1, the first exhaust heating gas flowrate measurement section B2, the flow rate setting section B3, the fuelgas supply section B4, the cell temperature measurement section B5, thegas temperature setting section B6, the cell temperature determinationsection B7 and the mode switching section B8. However, in thisembodiment, in addition to theses functions, the controller B alsoperforms the following functions: (1) measuring the temperature of themixed heating gas supplied to the cathode 13; and (2) determining thetemperature difference between the mixed heating gas supplied to thecathode 13 and the exhaust heating gas flowing through the return pipe17.

The programming and/or hardware of the controller B used to perform thefunction of measuring the temperature of the mixed heating gas suppliedto the cathode is referred to as the “cathode-supplied gas temperaturemeasurement section B15.” In the present embodiment, the temperature ofthe mixed heating gas is measured based on temperature data acquired bythe temperature sensor 63. The programming and/or hardware of thecontroller B used to perform the function of determining the temperaturedifference between the mixed heating gas supplied to the cathode 13 andthe exhaust heating gas flowing through the return pipe 17 is referredto as the “gas temperature difference determination section B16.” Inother words, the gas temperature difference determination section B16determines whether or not the temperature difference between the mixedheating gas supplied to the cathode 13 and the exhaust heating gasdischarged from the cathode 13 exceeds a predetermined value. Upondetermining that this temperature difference is outside of apredetermined range, the gas temperature setting section B6 resets thetemperature of the heating gas supplied to the cathode 13 so that thetemperature difference reverts back into the predetermined range.

The method of raising the temperature of a fuel cell that uses the fuelcell system A4 having the above-described configuration is describedwith reference to FIG. 12. In the present embodiment, the flow rates ofthe fuel and the air supplied to the first combustor 20 are regulatedbased on the flow rate and the temperature of the exhaust heating gasflowing through the return pipe 17 so that the heating gas supplied tothe cathode 13 reaches a predetermined temperature, similar to the fuelcell system A1 described above.

In step Sd1, the process of raising the temperature of the fuel cell 10for a startup operation is started, and the process advances to stepSd2.

In step Sd2, the exhaust heating gas is supplied back through the returnpipe 17.

In step Sd3, the temperatures of the fuel cell 10 and the exhaustheating gas are detected, measured and stored.

In step Sd4, the temperature of the heating gas supplied to the fuelcell 10 is set. Specifically, the temperature of the heating gassupplied to the fuel cell 10 is set based on the temperature of the fuelcell 10.

In step Sd5, a decision is made as to whether or not the temperaturedifference between the fuel cell 10 and the exhaust heating gas is equalto or greater than a predetermined value. Upon determining that thistemperature difference is equal to or greater than the predeterminedvalue, the process advances to step Sd6. Otherwise, if this temperaturedifference is not equal to or greater than the predetermined value, thenthe process advances to step Sd10. In other words, a decision is made asto whether or not the supplied heating gas temperature minus thetemperature of exhaust heating gas circulating through return pipe 17exceeds the predetermined value.

In step Sd6, the amount of combustion in the first combustor 20 isdetermined from the temperature and the circulated supplied amount ofthe exhaust heating gas.

In step Sd7, fuel and air are supplied to the first combustor 20 inamounts determined by calculation.

In step Sd8, new heating gas and exhaust heating gas are mixed andsupplied.

In step Sd9, a decision is made as to whether or not the fuel cell 10has reached a predetermined temperature. When the predeterminedtemperature is determined to have been reached, the process advances tostep Sd11, otherwise the process returns to step Sd2.

In step Sd10, in cases in which the predetermined value is exceeded,combustion is not performed in the first combustor 20, and exhaustheating gas alone is supplied through the return pipe 17 to the fuelcell 10. The upstream side of the fuel cell 10 is thereby cooled whilethe downstream side is heated, and the temperature can immediately berectified.

In step Sd11, the heating and temperature-raising operation is ended,and the system transitions to the normal operation mode.

A summary of the fuel cell system A4 according to the present embodimentis as follows. Specifically, when the temperature difference between theupstream and downstream sides of the fuel cell 10 has reached thepredetermined value, the heating gas produced in the first combustor 20is supplied to the fuel cell 10 without combining with the circulatinggas. In other words, a system of cooling the combustion gas supplied tothe fuel cell 10 is used, whereby the temperature difference between theupstream and downstream sides of the fuel cell 10 can be immediatelyresolved. The thermal stress created in the fuel cell 10 can be reduced,whereby the reliability of the fuel cell 10 during temperature elevationis improved. Furthermore, when the temperature of the supplied heatinggas is lowered in cases of an upstream/downstream temperaturedifference, the fuel cell 10 is cooled, and heat supplied to the fuelcell 10 is temporarily transferred to the heating gas.

At this time, if the exhaust heating gas is circulated as it is in thissystem, heat supplied to the fuel cell 10 is circulated back to the fuelcell 10, but in a conventional system which does not circulate exhaustheating gas, heat taken from the fuel cell 10 flows out of the system.Therefore, extra heat must be supplied in order to raise the temperatureof the fuel cell 10, which causes fuel consumption to be worse, butthere is no such worsening of fuel consumption during temperatureelevation in the present system, and upstream/downstream temperaturedifferences can be immediately resolved.

In the present embodiment, in view of improving reliability, a system isused in which the temperature of the heating gas is reduced when anupstream/downstream temperature difference has occurred, but otheroptions include using a system in which the temperature-raising rate isreduced, a system in which heating gas of the same temperature issupplied, or the like. At such a time, an appropriate predeterminedvalue (allowable value) of the upstream/downstream temperaturedifference is set.

In cases in which the temperature of the fuel gas is reduced, thepredetermined value is preferably set to a comparatively hightemperature because the temperature difference can be immediatelyreduced, and in cases in which the temperature-raising rate is reduced,the predetermined value is set to a comparatively low temperaturebecause the temperature difference is resolved at a slow rate.Furthermore, the upstream/downstream temperature difference can also beresolved by combining systems in which the temperature-raising rate isreduced at a first predetermined value, the temperature is maintained ata second predetermined value, the temperature is reduced at a thirdpredetermined value, etc.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. For example, in the present embodiment describedabove, an example is described in which an gas supplier 50 is providedto the return pipe 17, but instead of the gas supplier 50 beingprovided, the flow channel cross section of the discharge pipe 13 a andthe return pipe 17 can be designed so that the ratio of fuel gas to airin the first combustor 20 falls within the desired range, in view of thequantity of air flowing through the cathode 13 when the cell stack 15has reached a predetermined temperature.

Also, for example, the flow channel cross sections are not limited tobeing different from each other; another option is to appropriately setthe flow channel lengths of the discharge pipe 13 a and the return pipe17.

In the present embodiment, an example was presented in which the gassupplier was provided to the return pipe, but in another configuration,e.g., the gas supplier is shared with a circulation part of the anodeused during the normal operation mode. Specifically, pipes and switchingvalves can be appropriately connected and used accordingly so as tofunction as circulation device for the cathode during temperatureelevation and for the anode during normal operation.

An air blower was described as an example of a gas supplier, but anejector or the like can also be suitably used.

Thus, the present invention is not limited to the embodiments describedabove; modifications such as described hereunder can be made. Forexample, components that are shown directly connected or contacting eachother can have intermediate structures disposed between them. Thefunctions of one element can be performed by two, and vice versa. Thestructures and functions of one embodiment can be adopted in anotherembodiment. It is not necessary for all advantages to be present in aparticular embodiment at the same time. Every feature which is uniquefrom the prior art, alone or in combination with other features, alsoshould be considered a separate description of further inventions by theapplicant, including the structural and/or functional concepts embodiedby such feature(s). Thus, the foregoing descriptions of the embodimentsare provided for illustration only, and not for the purpose of limitingthe invention as defined by the appended claims and their equivalents.

What is claimed is:
 1. A fuel cell system comprising: a fuel cellincluding a solid electrolyte cell with an anode and a cathode, the fuelcell being configured to generate power by reacting ahydrogen-containing gas and an oxygen-containing gas; a first combustorarranged to selectively supply a heating gas to the cathode of the fuelcell; a second combustor arranged to selectively supply a heating gas tothe anode of the fuel cell; a first heating gas return channel arrangedto mix at least some exhaust gas discharged from the cathode with theheating gas from the first combustor such that a mixed heating gas ofthe exhaust gas from the cathode and the heating gas from the firstcombustor is supplied to the cathode; a second heating gas returnchannel arranged to mix at least some exhaust gas discharged from thecathode with the heating gas from the second combustor such that a mixedheating gas of the exhaust gas from the cathode and the heating gas fromthe second combustor is supplied to the anode; and a gas supplierconnected to the first heating gas return channel for supplying theexhaust gas from the cathode to mix with the heating gas from the firstcombustor.
 2. The fuel cell system according to claim 1, wherein thesecond heating gas return channel is arranged such that the exhaust gassupplied back to the anode is at least some of an excess portion of theexhaust gas discharged from the cathode that is not supplied back to thecathode.
 3. The fuel cell system according to claim 1, furthercomprising a first exhaust heating gas temperature measurement sectionarranged to measure a temperature of the exhaust gas flowing through thefirst heating gas return channel; a first exhaust heating gas flow ratemeasurement section arranged to measure a flow rate of the exhaust gasflowing through the first heating gas return channel; a first flow ratesetting section arranged to set flow rates of the hydrogen-containinggas and the oxygen-containing gas supplied to the first combustor suchthat the mixed heating gas supplied to the cathode reaches apredetermined temperature, the flow rates of the hydrogen-containing gasand the oxygen-containing gas supplied to the first combustor beingbased on the flow rate measured by the first exhaust heating gastemperature measurement section and the temperature measured by thefirst exhaust heating gas flow rate measurement section; and a fuel gassupply section arranged to supply the hydrogen-containing gas and theoxygen-containing gas to the first combustor at the flow rates set bythe flow rate setting section.
 4. The fuel cell system according toclaim 3, further comprising a cell temperature measurement sectionarranged to measure a temperature of the fuel cell; a gas temperaturesetting section arranged to set a temperature of the heating gassupplied from the first combustor to the cathode based on thetemperatures measured by the first exhaust heating gas temperaturemeasurement section and the cell temperature measurement section; a celltemperature determination section that determines whether the fuel cellhas reached an operation starting temperature; and a mode switchingsection arranged to switch from a temperature-raising mode to a normaloperation mode upon determining that the fuel cell has reached theoperation starting temperature.
 5. The fuel cell system according toclaim 4, wherein the gas temperature setting section is arranged to setthe temperature of the heating gas supplied from the first combustor toincrease over time to a target temperature.
 6. The fuel cell systemaccording to claim 1, further comprising a second exhaust heating gasflow rate measurement section arranged to measure a flow rate of theexhaust gas supplied to the anode by the second heating gas returnchannel; a second exhaust heating gas temperature measurement sectionarranged to measure a temperature of the exhaust gas supplied to theanode by the second heating gas return channel; and a second flow ratesetting section arranged to set flow rates of the hydrogen-containinggas and the oxygen-containing gas supplied to the second combustor suchthat a steam-to-carbon ratio and a temperature of the exhaust gassupplied to the anode reach predetermined values, the flow rates of thehydrogen-containing gas and the oxygen-containing gas supplied to thesecond combustor being set based on the flow rate measured by the secondexhaust heating gas flow rate measurement section and the temperaturemeasured by the second exhaust heating gas temperature measurementsection.
 7. The fuel cell system according to claim 3, furthercomprising a second exhaust heating gas flow rate measurement sectionarranged to measure a flow rate of the exhaust gas supplied to the anodeby the second heating gas return channel; a second exhaust heating gastemperature measurement section arranged to measure a temperature of theexhaust gas supplied to the anode by the second heating gas returnchannel; and a second flow rate setting section arranged to set flowrates of the hydrogen-containing gas and the oxygen-containing gassupplied to the second combustor such that a steam-to-carbon ratio and atemperature of the exhaust gas supplied to the anode reach predeterminedvalues, the flow rates of the hydrogen-containing gas and theoxygen-containing gas supplied to the second combustor being set basedon the flow rate measured by the second exhaust heating gas flow ratemeasurement section and the temperature measured by the second exhaustheating gas temperature measurement section.
 8. The fuel cell systemaccording to claim 4, further comprising a second exhaust heating gasflow rate measurement section arranged to measure a flow rate of theexhaust gas supplied to the anode by the second heating gas returnchannel; a second exhaust heating gas temperature measurement sectionarranged to measure a temperature of the exhaust gas supplied to theanode by the second heating gas return channel; and a second flow ratesetting section arranged to set flow rates of the hydrogen-containinggas and the oxygen-containing gas supplied to the second combustor suchthat a steam-to-carbon ratio and a temperature of the exhaust gassupplied to the anode reach predetermined values, the flow rates of thehydrogen-containing gas and the oxygen-containing gas supplied to thesecond combustor being set based on the flow rate measured by the secondexhaust heating gas flow rate measurement section and the temperaturemeasured by the second exhaust heating gas temperature measurementsection.
 9. The fuel cell system according to claim 1, furthercomprising a flow rate regulation valve arranged on the discharge sideof the cathode downstream from the first heating gas return channel, theflow rate regulation valve being connected to the second heating gasreturn channel to control an amount of the exhaust gas discharged fromthe cathode that is mixed with the heating gas from the secondcombustor.
 10. The fuel cell system according to claim 6, furthercomprising a flow rate regulation valve arranged on the discharge sideof the cathode downstream from the first heating gas return channel, theflow rate regulation valve being connected to the second heating gasreturn channel to control an amount of the exhaust gas discharged fromthe cathode that is mixed with the heating gas from the secondcombustor, the second exhaust heating gas temperature measurementsection including a temperature sensor disposed on the flow rateregulation valve.
 11. A solid oxide fuel cell temperature raising methodcomprising: producing a heating gas in a first combustor that receives ahydrogen-containing gas and an oxygen-containing gas; producing aheating gas in a second combustor that receives a hydrogen-containinggas and an oxygen-containing gas; recirculating a first portion ofexhaust gas discharged from a cathode of a fuel cell back to an intakeside the cathode by using a gas supplier that causes the exhaust gas tomix with the heating gas from the first combustor to produce a mixedheating gas that flows through the cathode; recirculating a secondportion of the exhaust gas discharged from the cathode of the fuel cellto an intake side of an anode of the fuel cell such that the exhaust gasmixes with the heating gas of the second combustor to produce a mixedheating gas that flows through the anode, the recirculation of the firstand second portions of the exhaust gas discharged from the cathode beingexecuted before fuel cell operation is started such that the fuel cellheats to an operating temperature of the fuel cell; measuring atemperature of the first portion of the exhaust gas being recirculatedfrom the cathode; measuring a flow rate of the first portion of theexhaust gas being recirculated from the cathode; setting flow rates ofthe hydrogen-containing gas and the oxygen-containing gas supplied tothe first combustor such that the mixed heating gas supplied to thecathode reaches a predetermined temperature, the flow rates of thehydrogen-containing gas and the oxygen-containing gas supplied to thefirst combustor being set based on the measured temperature and themeasured flow rate of the first portion of the exhaust gas dischargedfrom the cathode; and supplying the fuel and the oxygen-containing gasto the first combustor at the set flow rates.
 12. The solid oxide fuelcell temperature raising method according to claim 11, furthercomprising: measuring a temperature of the second portion of the exhaustgas being recirculated from the cathode; measuring a flow rate of thesecond portion of the exhaust gas being recirculated from the cathode;setting flow rates of the hydrogen-containing gas and theoxygen-containing gas supplied to the second combustor such that asteam-to-carbon ratio and a temperature of the exhaust gas recirculatedto the anode reach predetermined values, the flow rates of thehydrogen-containing gas and the oxygen-containing gas supplied to thesecond combustor being set based on the measured temperature and themeasured flow rate of the second portion of the exhaust gas; andsupplying the fuel and the oxygen-containing gas to the second combustorat the set flow rates.